Power plants that use pulverized coal as the combustible energy source produce, as a by-product, microsized fused particles of the ash contained in the coal. These particles generally take the form of glassy microspheres, and are called fly ash. The ash content of coals used for power generation generally varies from around one to ten percent of the weight of the coal. In some modern plants equipped to handle it, the ash content is even up to twenty percent. This ash generally is composed primarily of silica and alumina which average 50% to 60%, and 20% to 25%, respectively, of the weight of the ash. These two components are generally present in the coal, and make up the major portion of the ash. In addition to the alumina and silica, coal ash usually contains a significant amount of iron oxide, which is usually up to a few percent of the ash weight, but can range up to 10% or more. Minor amounts of many other oxides such as the oxides of titanium, calcium, magnesium and potassium, are usually present. These ash compositions are generally good glass formers.
Upon passing through the boiler furnace of a power plant the coal particles burn and the ash content of each particle fuses. This “fly ash” product is usually collected by electrostatic precipitators, and is sold in large tonnages for concrete highway construction, high rise building construction, and similar uses.
A small fraction of fly ash, usually around one percent of the ash produced, is in the form of hollow microspheres and will float on water. Those power plants which have settling water ponds are able to separate the floating fly ash from the denser fly ash, and recover it as a hollow glass microsphere product which is called “cenospheres.” This product is generally about 0.7 to 0.8 grams per cubic centimeter in average particle density, and is therefore much more valuable than the denser fly ash. It is sold for use as a filler in light weight products such as plastics, putties, and concrete.
In contrast to the miniscule yield of cenospheres in fly ash the technology of commercial hollow glass microspheres has become advanced enough so that nearly 100% of the precursor feed particles fed into “bubble” formers will float on water. Hollow microspheres of this type are marketed by a number of companies, and a range of useful compositions for the microspheres has been taught in published literature. Blowing agents are incorporated into the glass so that, when heated above the fusion temperature of the glass, the gas is released to blow the particle into a bubble. These “bubbles” are produced with particle densities much lower than that of cenospheres, and usually average from around 0.4 down to around 0.1 grams per cubic centimeter. They therefore have a much higher market value than cenospheres, and sell for a higher price. These commercial bubbles are generally colorless, in contrast to the usual dark color of cenospheres.